[Discuss] Trying to connect to internet in Debian

[Kent Borg]

On 1/16/26 5:01 PM, Randall Rose wrote:
> Of course, if you can recommend a way of finding out which of the thousands of packages that currently are or might later be on my machine could be listening to the network, I would appreciate hearing.  That would be useful information.  I just don't know it.

My approach is to do a port scan of myself, to check what network ports 
are open and listening for incoming connections using "nmap", pretty 
standard program.

First, find out what your network address, say it is 10.1.2.3, then run 
something like:

   nmap -A -T3 10.1.2.3

When I run it against my laptop I get:

> root at theseion:/home/kentborg# nmap -A -T3 10.0.0.184
> Starting Nmap 7.95 ( https://nmap.org ) at 2026-01-16 19:06 PST
> Nmap scan report for theseion.lan (10.0.0.184)
> Host is up (0.000058s latency).
> All 1000 scanned ports on theseion.lan (10.0.0.184) are in ignored states.
> Not shown: 1000 closed tcp ports (reset)
> Too many fingerprints match this host to give specific OS details
> Network Distance: 0 hops
>
> OS and Service detection performed. Please report any incorrect 
> results at https://nmap.org/submit/ .
> Nmap done: 1 IP address (1 host up) scanned in 1.72 seconds
> root at theseion:/home/kentborg#

Nothing listening, very boring.

Note that if you do an nmap of "localhost" you will see what is visible 
from within the machine looking at itself, and can get different 
answers, you need to specify your IP address to get what others see. 
Even then it is probably cleaner to do the nmap from a different machine 
to really get the outside world's perspective.


When I run at against one of my e-mail servers, much more interesting.

I won't paste in the whole output, but I see something is listening on:

- port 22  sshd, good, I use that everyday
- port 25  smtpd, good, needed for incoming e-mail
- port 145  imapd, good, for users to read e-mail
- port 465  ssl/smtp, good, also for users to send e-mail
- port 993  imapd, good, for users to read e-mail, maybe I don't need both.

That's it.


When I do a scan of my web server I see:

- port 22  sshd again
- port 25  smtpd again, though it accepts mail for almost nothing
- port 80  web
- port 443  encrypted web

Again, just what I expect.


If I were to put on a firewall I would have to let those through, and I 
would block attempts to talk to…all the other possible ports, that 
nothing is listening to anyway.

-kb